Radio frequency identification tags and radio frequency identification tag systems are known, and find numerous uses. For example, radio frequency identification tags are frequently used for personal identification in automated gate sentry applications protecting secured buildings or areas. Information stored on the radio frequency identification tag identifies the person seeking access to the secured building. A radio frequency identification tag system conveniently provides for reading the information from the radio frequency identification tag at a small distance using radio frequency (RF) data transmission technology. Most typically, the user simply holds or places the radio frequency identification tag near a base station that transmits an excitation signal to the radio frequency identification tag powering circuitry contained on the radio frequency identification tag. The circuitry, responsive to the excitation signal, communicates the stored information from the radio frequency identification tag to the base station, which receives and decodes the information. In general, radio frequency identification tags are capable of retaining and, in operation, transmitting a substantial amount of information--sufficient information to uniquely identify individuals, packages, inventory and the like.
The radio frequency identification tag is also capable of receiving and storing information. In a read/write application, the base station is not only capable of sending an excitation signal and receiving a response from the radio frequency identification tag, but it is also capable of sending a data, or write, signal to the radio frequency identification tag. The radio frequency identification tag receives the write signal, which may contain data to be stored within the tag, a code or a command. Depending on the type of write signal, the radio frequency identification tag responds accordingly, such as by storing the data or acting upon the command.
A typical technology for powering and reading a radio frequency identification tag is inductive coupling or a combination of inductive power coupling and capacitive data coupling. Inductive coupling utilizes a coil element in the radio frequency identification tag. The coil element is excited (or "energized") by an excitation signal from the base station to provide power to the radio frequency identification tag circuitry. The radio frequency identification tag coil, or a second tag coil, may be used to transmit and receive the stored information between the radio frequency identification tag and the base station. Radio frequency identification tags relying on inductive coupling are sensitive to orientation of the radio frequency identification tag with respect to the base station since the field created by the excitation signal must intersect the coil element at substantially a right angle for effective coupling. Read ranges for inductively coupled devices are generally on the order of several centimeters. Longer read distances are desirable, and for certain applications, such as electronic animal identification, baggage tracking, parcel tracking and inventory management applications, are necessary.
Another technology for powering and reading radio frequency identification tags is electrostatic coupling such as employed in the radio frequency identification tag systems and radio frequency identification tags disclosed in the above referenced applications. These systems advantageously provide for substantially increased read/write distances over those available in the prior art. Another advantage derived from the use of the systems and tags therein disclosed is that the user need not bring the radio frequency identification tag in close proximity to a base station or to substantially orient the tag with respect to the base station. It is therefore possible to incorporate the antenna elements of the base station into, for example, a doorway or a vestibule, a package conveyer or an article sorting system, and to energize the tag and read the tag information at a greater distance.
To couple either the inductive or electrostatic signals between the base station and the radio frequency identification tag, the tag necessarily includes an antenna having at least one and frequently two antenna elements. Typically, a tag circuit chip and the antenna are electrically coupled and bonded to a tag substrate. The tag may also include additional components, for example, resistors, capacitors, inductors, etc. that must also be electrically coupled to the tag circuit chip and/or the antenna. Conventional tag design provides conductive traces formed on a substrate with the tag circuit chip, components and antenna bonded to the substrate and electrically coupled to the conductive traces. Wire bonding is a common technique for providing an electrical couple between the conductive pads on the tag circuit chip and/or the component and the conductive traces. Alternatively, "flip" chip technology provides raised conductive regions ("bumped pads") on the tag circuit chip (and similarly on the electrical components). The "flip" chip, during assembly, is inverted and positioned to the substrate with the bumped pads aligning with and electrically coupling to the conductive traces. A conductive adhesive may be used between the bumped pads and the conductive traces to ensure a good electrical couple as well as to supplement the mechanical adhesion of the tag circuit chip to the substrate.
Size, and particularly thickness, of the radio frequency identification tag is important, and it is desirable to maintain overall thickness at less than 0.5 millimeter (mm). To provide thinner tags, it is proposed to form the conductive traces by printing conductive ink onto the substrate, which is typically paper or synthetic paper. This all but eliminates wire bonding as a joining technology as it generally requires a relatively rigid substrate and is thus not well suited for coupling to traces printed on a paper substrate. Wire bonding also unacceptably adds to the tag thickness. For example, a typical wire bond may add as much as 0.07-0.18 mm to the tag thickness in wire bond loop height.
Flip chip technology is more suited for coupling to printed traces, but it too increases the thickness of the tag, i.e., the bumped pads provide a thicker overall profile for the tag circuit chip. Flip chip technology adds cost to the tag circuit chip in the forming of the bumped pads and requires the use of a backfill adhesive for good mechanical adhesion to the substrate. Flip chip technology also requires high cost production equipment to pick, invert and place the tag circuit chip to the substrate. Namely, the tag circuit chip must be first inverted from the fabrication wafer and then precision aligned to make the blind attachment to the substrate. Use of an anisotropic adhesive alleviates some of the alignment difficulties and eliminates the need for backfill adhesive, but at substantially increased cost over isotropic adhesives.
Thus, there is a need for an improved radio frequency identification tag.